What is it you are trying to do?
This may help. Download plugin ress and look at source code.
Edit: no, my mistake session_start is wrong hook. Sorry
Another suggestion, point return params of all login pages to an empty template where you run a plugin to test for T&C = done. If so, redirect here. If not, redirect there.
You can also use group_ids to redirect.
{exp:member:login_form return="template_group/my_login_redirect_template"}
For example
{if logged_in}
{exp:my_login_plugin}
{redirect='pages/terms_conditions'}
{/exp:my_login_plugin}
{/if}
OR
{exp:switchee variable = "{group_id}" parse="inward"}
{case value="9"}
{redirect='pages/terms_conditions'}
{/case}
{case value="6"}
{redirect='template_name/member'}
{/case}
{case default="Yes"}
{redirect='template_name/welcome'}
{/case}
{/exp:switchee}